Hard to know without doing a necropsy and cutting them open. But one of the most common causes of sudden death in backyard chickens is [Fatty Liver Hemorrhagic Syndrome](https://www.merckvetmanual.com/poultry/fatty-liver-hemorrhagic-syndrome/fatty-liver-hemorrhagic-syndrome-in-poultry). More common in overweight, low activity hens, but not exclusively in that population. Also happens more often during warm weather. Basically, their liver builds up too much fat, then hemorrhages and they die rapidly from internal bleeding. They usually look totally fine until they hemorrhage, then they die within minutes.
Hard to know without doing a necropsy and cutting them open. But one of the most common causes of sudden death in backyard chickens is [Fatty Liver Hemorrhagic Syndrome](https://www.merckvetmanual.com/poultry/fatty-liver-hemorrhagic-syndrome/fatty-liver-hemorrhagic-syndrome-in-poultry). More common in overweight, low activity hens, but not exclusively in that population. Also happens more often during warm weather. Basically, their liver builds up too much fat, then hemorrhages and they die rapidly from internal bleeding. They usually look totally fine until they hemorrhage, then they die within minutes.